A device for producing cryogenic refrigeration of the type for which the present invention is ideally suited is disclosed and claimed in U.S. Pat. No. 3,620,029. Patentee discloses a displacer-expander type refrigerator where the displacer is cycled against a volume of surge fluid driven through an orifice so that external driving means for the displacer are unnecessary. Work is expended by forcing the surge gas through the orifice into a surge volume chamber whereby the heat generated by such action can be removed by suitable heat exchange. The device of the U.S. Pat. No. 3,620,029 includes a ported rotary valve for admitting high-pressure fluid to the variable volume chamber or cold end of the refrigerator and exhausting low pressure expanded gas from the refrigerator. The device according the U.S. Pat. No. 3,620,029 may have more than one stage, and most current devices of this type employ two-stage refrigeration such that, at the first stage of the refrigerator, temperatures of between 35.degree. and 85.degree. Kelvin (K.) are achieved when helium is the working fluid and temperatures of 10.degree. to 20.degree. K. are achieved at the second stage with the same working fluid.
Refrigerators of the type disclosed in the U.S. Pat. No. 3,620,029 are ideally suited for use in superconducting magnets and other superconducting devices. In addition, whole body nuclear magnetic resonance (NMR) scanners, magnetic separators and Josephson junction devices require cryostats employing liquid helium cooling. A refrigerator according to the U.S. Pat. No. 3,620,029 can be used to cool radiation shields and reliquefy helium boiloff in such cryostats and to minimize helium boiloff in such devices.
In using such devices with NMR equipment, it has been found that the conventional device with the motor valve disc and expander as a single unit tended to cause magnetic disturbances in the NMR device. Separating the valve mechanism and motor from the displacer by use of long gas lines interconnecting the two led to substantial refrigeration losses because of the increased void volume in the refrigeration system.